Description

State-of-the-art ultrashort intense light sources allow us to observe quantum electron dynamics in various materials. One of the most fundamental processes for laser processing in dielectrics is electron excitation via nonlinear processes due to the intense laser pulse. Fingerprints of the electron excitation dynamics driven by the intense pulse emerge as high-harmonic generation (HHG) spectra and photoelectron energy spectra. While HHG in the gas phase is well understood by the three-step model[1], a comprehensive mechanism of solid-state HHG is still under active research[2,3].

Elucidating the origin of the difference improves the fundamental comprehension of the nonlinear optical absorption in solid-state materials. We investigate how the HHG spectrum changes between the gas-phase and the solid-state systems by increasing inter-atomic distance (IAD) of a one-dimensional model crystal from a solid-like system to a gas-like system.

We solve time-dependent Schroedinger equations for independent electrons within the velocity gauge for each Bloch orbital index n and crystal momentum k. The Hamiltonian contains local effective potential,

Veff(r)=-v0[1+cos(2πr/a)] (0<r<a+b)

where a=0.42 nm, v0=10 eV (0<r<a) or v0=0 eV (a<r<a+b).

a is an atomic potential well width, b is a variable to control IAD or a+b.

We consider a driving laser pulse with 800 nm wavelength, 9 fs FWHM pulse duration, and 20 V/nm peak intensity. High-harmonic spectra are calculated as power spectra of current density, obtained from the wave function ψn,k(r,t).

We find that the HHG spectra show two transitions at small IAD (~0.7 nm) and large IAD (≲ 50 nm). Our analyses reveal the following two physical pictures corresponding to the transitions. The former transition is due to a static effect induced by a change of electronic structure which affects the excitation probability of valence electrons. The latter is due to a dynamic effect related to the electronic dynamical motion under the intense laser field.

[1] P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993).

[2] K. K. Hansen, D. Bauer, and L. B. Madsen, Phys. Rev. A 97, 043424 (2018).

[3] G. L. Breton, A. Rubio, and N. Tancogne-Dejean, Phys. Rev. B 98, 165308 (2018).

Contributing Authors

  • Mizuki Tani
    The University of Tokyo
  • Yasushi Shinohara
    The University of Tokyo
  • Takeshi Sato
    The University of Tokyo
  • Kenichi L Ishikawa
    The University of Tokyo
Mizuki Tani
The University of Tokyo
Track: Laser Nanomanufacturing
Session: On-demand Only
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Keywords

  • High-Order Harmonic Generation
  • Nonlinear Optical Absorption
  • Quantum Mechanics
  • Transient Electron Dynamics
  • Ultrafast Phenomena